1. Field of the Invention
This invention relates to a method of oxidizing combustible materials in a high temperature water oxidation apparatus. More particularly, this method provides for an iridium contact area on the apparatus so as to alleviate the effect of corrosive material present in the process stream.
2. Description of the Related Art
The process of "wet oxidation" has been used for the oxidation of compounds in an aqueous stream for some time. Generally, it involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous stream at elevated temperatures and pressures, with the resultant "combustion" of oxidizable materials directly within the aqueous phase. This wet oxidation process is characterized by operating pressures of 25 to 250 bar (440 to 3630 psia) and operating temperatures of 150.degree. to 370.degree. C. At these conditions, the gas phase oxidation is quite slow and the majority of the oxidation reaction is carried out in the liquid phase. Thus, the reactor operating conditions are typically maintained at or about the saturation point of water, such that at least a part of the water is present in liquid form. This wet oxidation process has several drawbacks. First, it is unsuitable to adequately handle refractory compounds. Second, it is characterized by slow reaction times. Third, due to the low temperature of the process, heat recovery is limited.
In light of such limitations, aqueous oxidation processes were extended to higher temperatures and pressures. In U.S. Pat. No. 2,944,396 to Barton et al., the addition of a second oxidation stage after a wet oxidation reactor is taught. Unoxidized volatile combustibles which accumulate in the vapor phase of the first stage wet oxidation reactor are oxidized in a second stage, which is operated at temperatures above the critical temperature of water of about 374.degree. C. U.S. Pat. No. 4,292,953 to Dickinson, discloses a modified wet oxidation process for power generation from coal and other fuels in which, as heat is liberated by combustion, the reaction mixture exceeds the critical temperature of water, with operating pressures of about 69 bar (1000 psi) to about 690 bar (10,000 psi) spanning both the sub- and supercritical water pressure ranges. U.S. Pat. No. 4,338,199 to Modell, discloses a wet oxidation process which has come to be known as supercritical water oxidation (SCWO) because in some implementations oxidation occurs essentially entirely at conditions supercritical in temperature (&gt;374.degree. C.) and pressure (&gt;about 3200 psi or 220 bar) for water. SCWO at 500.degree.-650.degree. C. and 250 bar has been shown to give rapid and near complete oxidation of organic compounds. A related process known as supercritical temperature water oxidation (STWO) can provide similar oxidation effectiveness for certain feedstocks but at lower pressure. This process has been described in U.S. Pat. No. 5,106,513 to Hong, and utilizes temperatures in the range of 600.degree. C. and pressures between 25 and 220 bar.
These aqueous oxidation processes will hereinafter be referred to collectively as "hydrothermal oxidation" ("HTO") if carried out at a temperature in the range of about 374.degree. C. to about 800.degree. C. and pressures above about 25 bar. Characteristics of the HTO process will be described below for the specific case of SCWO, though other HTO environments will have much in common.
SCWO may be compared to incineration processes since its efficiency towards oxidizable materials is almost 100%. Indeed, much of the process development to date in SCWO has been directed toward treatment of sludges or toxic and hazardous wastes. Such materials could likewise be subjected to an incineration process.
Other potential feedstocks for the SCWO process include those wastes which are currently being handled by deep well injection techniques and wastes which have either been accumulated or spilled, including mixed radioactive/organic wastes. Due to the wide variety of potential feedstocks, the species of interest to the SCWO process span virtually the entire periodic table.
The oxidation occurring in the SCWO process is comparable to incineration in that carbon and hydrogen form the conventional combustion products CO.sub.2 and H.sub.2 O. Halogenated hydrocarbons may form strong acids, for example, chlorinated hydrocarbons (CHCs) may give rise to HCl. The formation of strong halogen acids may lead to acid corrosion problems for the processing equipment. In the past, alkali has been added to mitigate acid corrosion problems.
In contrast to normal combustion, which forms SO.sub.2, the final product of sulfur oxidation in SCWO is sulfate anion. As in the case of chloride, alkali may be intentionally added to avoid high concentrations of sulfuric acid. Similarly, the product of phosphorus oxidation is phosphate anion.
While it is frequently desirable to neutralize oxidation product anions via alkali addition, neutralization of cations is not usually necessary. Feedstocks containing excess noncombustible cations are generally self-neutralized. by the CO.sub.2 which evolves from oxidation. For example, a stream containing organic sodium salts will yield sodium carbonate as a product. Ammonium, another common cation, is converted to water and nitrogen (N.sub.2) or nitrous oxide (N.sub.2 O) in the SCWO process, and so does not require neutralization.
A key advantage of SCWO over incineration is the lack of NO.sub.x formation due to the relatively low temperature of operation. Oxidized forms of nitrogen, e.g., organic nitro-compounds and nitrate anion, have been found to form N.sub.2 or N.sub.2 O. When air is used as the oxidizing agent, N.sub.2 passes through the system as an inert.
While chemical equilibria under SCWO conditions has been fairly well characterized, much remains to be learned about chemical kinetics and reaction mechanisms. The situation is complicated by the wide range of densities which may exist in supercritical water systems. At typical reactor conditions of 500.degree. to 600.degree. C., the supercritical phase density is on the order of 0.1 g/cc. Reaction mechanisms are of the free radical type as with normal combustion, but can be greatly affected by the higher density and water concentration which characterize SCWO conditions. On the other hand, at temperatures closer to the critical point, or in dense supercritical brine phases, densities of 0.5 to 1 g/cc are obtained and ionic reaction mechanisms may dominate.
As indicated by the density employed under typical reactor conditions (0.1 g/cc), the distance between water molecules is considerably greater than the distance between molecules in normal liquid water. The disruption of hydrogen bonding causes the water molecules to lose the molecular ordering which is responsible for many of the properties of normal liquid water. In particular, solubility behavior approximates that of high pressure steam rather than that of liquid water. Smaller polar and nonpolar organic compounds, with relatively high volatility, exist as vapors at typical SCWO conditions, and hence are completely miscible with the supercritical water. Gases such as N.sub.2, O.sub.2, and CO.sub.2 show similar miscibility. Larger organic compounds, such as polymers, pyrolyze or hydrolyze to smaller molecules at typical SCWO conditions, thereby resulting in solubilization via chemical reaction. The loss of bulk polarity of the water phase has striking effects on normally water-soluble salts, as well. No longer readily solvated by water molecules, they precipitate out as solids or dense brines. The small salt residual which is soluble in the supercritical phase is largely present in molecular form, e.g., as NaCl molecules. Heavy metal oxides, having low solubility in liquid water, retain their low solubility at supercritical water conditions. Exceptions exist and high solubilities occur, however, when a metal forms a volatile salt or oxide at reactor temperatures.
The characterization of solubility behavior in the preceding paragraph has been expressed in relation to pure supercritical water. In actual SCWO systems, this behavior may be greatly altered by the presence of large quantities of gases and salts. In many applications, for example, the mass of "noncondensible" gases in the reactor may exceed the mass of water. The presence of noncondensible gases and salts in the SCWO reactor encourages the separation of phases and is similar to the "salting out" phenomenon of gases from solution.
The complexity and uniqueness of the SCWO environment, combined with the elevated temperature and pressure requirements, presents a significant challenge in the selection of materials of construction for commercial applications.
While stainless steel has proven suitable for research in dealing with mixtures of water, oxygen, and hydrocarbons, commercial systems are required to handle a variety of acidic and alkaline streams, as well as streams containing a significant quantity of salts. High nickel alloys, in particular Alloy C276 and Alloy 625, have been used in testing. However, unacceptably high corrosion rates are observed with these alloys for many streams of interest. Furthermore, prolonged exposure to and cycling of these materials at reactor temperatures leads to a degradation of their mechanical properties. Both alloys are subject to embrittlement, thereby leading to the increased possibility of cracking and catastrophic failure.
A number of metals, alloys and ceramics have been tested. Recently, U.S. Pat. No. 5,358,645 (herein incorporated by reference) disclosed the use of zirconia based ceramics for the contact surface area of an apparatus for high temperature water oxidation of combustible materials. However, the brittleness of ceramics and their sensitivity to thermal shock limits the situations in which they are useful.
Thus, the presently known materials employed in hydrothermal oxidation reactors suffer from high corrosion rates, poor resistance to thermal cycling, and poor service life.
A need exists therefore for a material or a coating which demonstrates greater resistance to hydrothermal oxidation conditions. In particular, a need exists for a material or coating which may be commercially useful for containing the corrosive environment of the hydrothermal oxidation reactor.